Heretofore, a method for manufacturing a three-dimensional shaped object by irradiating a powder with a light beam has been known (such method can be generally referred to as a “selective laser sintering method”). Such method can produce a three-dimensional shaped object with a plurality of solidified layers stacked integrally by repeating the step (i) of forming a solidified layer by irradiating a predetermined portion of a powder layer with a light beam, thereby allowing sintering of the predetermined portion of the powder or melting and subsequent solidification thereof, and the step (ii) of forming another solidified layer by newly forming a powder layer on the resulting solidified layer, followed by similarly irradiating the powder layer with the light beam (see JP-T-01-502890 or JP-A-2000-73108). The three-dimensional shaped object thus obtained can be used as a metal mold in a case where inorganic powder materials such as a metal powder and a ceramic powder are used as the powder material. While on the other hand, the three-dimensional shaped object can be used as a model or replica in a case where organic powder materials such as a resin powder and a plastic powder are used as the powder material. This kind of technology makes it possible to produce the three-dimensional shaped object with a complicated the contour shape in a short period of time.
According to the selective laser sintering method, a three-dimensional shaped object is often manufactured in a chamber maintained under an inert atmosphere from the viewpoint of the prevention of oxidation or the like. By way of the case of using a metal powder as a powder material and using the resulting three-dimensional shaped object as metal mold, as shown in FIG. 1, a powder layer 22 with a predetermined thickness t1 is firstly formed on a base plate for shaped object 21 (see FIG. 1(a)) and then a predetermined portion of a powder layer 22 is irradiated with a light beam to form a solidified layer 24 on base plate for shaped object 21. Then, a powder layer 22 is newly provided on the solidified layer 24 thus formed and is irradiated again with the light beam to form another solidified layer. In this way, when the solidified layer is repeatedly formed, it is possible to obtain a three-dimensional shaped object with a plurality of solidified layers 24 stacked integrally (see FIG. 1(b)). The solidified layer corresponding to a bottom layer can be formed in a state of being adhered to the surface of the base plate for shaped object. Therefore, the three-dimensional shaped object and the base plate for shaped object are mutually integrated. The integrated “three-dimensional shaped object” and “base plate for shaped object” can be used as a metal mold as they are.
Herein, the three-dimensional shaped object is manufactured by irradiation with a light beam and is therefore considerably influenced by heat attributable to the light beam. Specifically, the portion to be irradiated of the powder layer is once melted to become a molten state, and then a solidified layer is formed when such portion is solidified. However, a shrinkage phenomenon can occur upon the solidification. In particular, the shrinkage phenomenon occurs when the molten powder is solidified due to a cooling thereof (see FIG. 2(a)). While on the other hand, a base plate for shaped object 21, which supports the solidified layer (i.e., three-dimensional shaped object), is a rigid body made of a steel material or the like and is distant from the position to be irradiated with the light beam. Therefore, the base plate for shaped object 21 is substantially less likely to be influenced by heat attributable to the light beam. As a result, an upward warping force (moment) is generated in the three-dimensional shaped object 24 on the base plate for shaped object. When this upward warping force exceeds a certain limit, there occurs a phenomenon in which the three-dimensional shaped object 24 peels from the base plate for shaped object 21 upon manufacturing, as shown in FIG. 2(b). The upward warp of the three-dimensional shaped object and the peeling thereof from the base plate are not desirable since it becomes difficult to manufacture a desired three-dimensional shaped object. For example, when the three-dimensional shaped object (i.e., solidified layer) warps upward, it becomes impossible to achieve a shape accuracy of the resulting three-dimensional shaped object. Also, when it becomes impossible to newly provide a powder layer with a predetermined thickness on the solidified layer due to the upward warping of the solidified layer (e.g., when the solidified layer warps upward to the extent more than the thickness of a powder layer to be newly provided), it will become impossible to uniformly provide the new powder layer by the squeegee process.
A manufacturing method as described in JP-T-8-504139 has been proposed as the method in which consideration is made on shrinkage of a three-dimensional shaped object. According to such manufacturing method, a three-dimensional shaped object is manufactured so that an inner core portion and an outer shell portion are separately formed (see FIG. 17). Since the outer shell portion has a solidified density higher than that of the inner core portion, a formation of the outer shell portion requires a high energy and also takes a longer time. In the invention of JP-T-8-504139, consideration is not made on a final use application of the shaped object and the outer shell portion, and only forms a “shell” as its name suggests. Therefore, in the invention of JP-T-8-504139, the entire periphery of the three-dimensional shaped object is coated with a material with a generally uniform thickness (see FIG. 17) and it is by no means satisfactory from the viewpoint of the manufacturing cost and time.